Ferritic stainless steels with improved cold-heading characteristics

ABSTRACT

A METHOD FOR IMPROVING THE COLD-HEADING CHARACTERISTICS OF FERRITIC STAINLESS STEELS. A COMPLEX EQUATION SHOWS THE INTERRELATION OF THE VARIOUS ALLOYING ELEMENTS. WITHIN A COMPOSITIONAL ARANGE SIMILAR TO THAT OF TYPE 430 STEEL, THE COLD-HEADING CHARACTER, AS REPRESENTED BY LOWERED $ VALUES, MAY BE IMPROVED BY INCREASED AMOUNTS OF MN AND MO AND DECREASED AMOUNTS OF SI. DUE TO THE INTERACTION BETWEEN C AND CR, IT IS BENEFICIAL TO EMPLOY EITHER HIGH C IN COMBINATION WITH LOW CR OR LOW C IN COMBINATION WITH HIGH CR.

P 26, 1972 K. G. BRICKNER ETAL 3,694,192

FERRITIC STAINLESS STEELS WITH IMPROVED COLD-HEADING CHARACTERISTICS Filed Aug. 11, 1970 CARBON 00MP0s/r/0/v 4m 2' Mo /.5 020 0/0 0.02

/a.5% Cr INVENTOR KENNETH G. BR/C/(NER A Homey United States Patent 'Oflice Patented Sept. 26, 1972 US. Cl. 75-126 B 13 Claims ABSTRACT OF THE DISCLOSURE A method for improving the cold-heading characteristics of ferritic stainless steels. A complex equation shows the interrelation of the various alloying elements. Within a compositional range similar to that of Type 430 steel, the cold-heading character, as represented by lowered '17 values, may be improved by increased amounts of Mn and Mo and decreased amounts of Si. Due to the interaction between C and Cr, it is beneficial to employ either high C in combination with low Cr or low C in combination with high Cr.

Stainless steel fasteners, such as bolts and screws, are used extensively in the building construction, chemical, and automotive industries. These fasteners are generally produced by a cold-heading (forging) operation. Materials that work harden readily are ditlicult to cold-head, in comparison to materials that exhibit low strain hardening rates. The strain-hardening character of a metal may be described by its strain-hardening coefficient, n, in the Ludwik equation r=Ke which closely describes the plastic portion of the true stress-true strain curves of many metals (i.e., the ferritic stainless steels). Therefore, for good cold-heading properties, low strain hardening coefficients are desired. Relatively small changes in n have a marked effect on the ease of cold-heading; thus, AISI Type 304 stainless steel, which exhibits an average n value of about 0.49, is considerably more difiicult to cold-head than AISI Type 305 stainless, with an average n value of 0.45. In comparison to the austenitic stainless steels, ferritic stainless steels, suchas AISI Type 430, exhibit n values of about 0.19-0.21 and are therefore easier to cold-head. However, as stainless steel fasteners become more complex with intricate configurations, and as cold-heading is conducted at higher and higher speeds, the need for ferritic stainless steels, which are even easier to cold-head, has increased.

It is therefore an object of this invention to provide annealed, ferritic stainless steel rod and wire with superior cold-heading properties.

It is another object of this invention to provide a method for tailoring the cold-heading characteristics of ferritic stainless steels for particular fastener applications.

It is a further object of this invention to provide annealed ferritic stainless steels with an average strainhardening exponent, E, no greater than 0.17.

These and other objects will be more apparent, when read in conjunction withe the appended claims and the detailed description in which:

FIG. 1 shows the interaction of carbon and chromium and their eifects on E value.

Shown in Table I are the compositions of the 34 steels that were prepared as induction-furnace heats during the development of the invention. These steels can also be made by conventional practice in an electric furnace. All heats were melted under an argon cover and cast into 3-inch-thick by S-inch-wide by 14-inch-high slab-type molds for this study. (Slab-type molds were used because it was desired to produce strip so that n values in the longitudinal, transverse and diagonal (45 degrees) directions could be determined and an average It value,

H, could be calculated. The average n value, n, is defined as follows:

where 0, 45, and 90 are degrees from the rolling direction of sheet or strip.) The surfaces of each slab-type ingot were conditioned by machining. The slabs were hot rolled at 2150 F. to 0.160-inch-thick strip. The strip-finishing temperature was about 1500 F. Hot-rolled strip from each heat was given a simulated box anneal at 1450 F. for six hours and furnace cooled. The annealed strip was shot-blasted to remove scale and then cold-rolled to 0.080-inch-thick strip. The cold-rolled strip was then annealed in salt for 15 seconds at 1450 F. and air cooled. The salt-annealed 0.080-inch-thick strip was cleaned with a detergent and cold-rolled to 0.040-inchthick strip, which was again annealed in salt for two minutes at 1450 F. and cooled. These latter annealing treatments simulated commercial continuous annealing. The processing cycle from ingot to cold-rolled and annealed strip simulated closely that which is given commercially produced AISI Type 430 stainless steel.

From the cold-rolled and annealed 0.040-inch-thick strip from each of the 34 heats, six tension-test specimen blanks were sheared. Two blanks were sheared parallel to the rolling direction (longitudinally), two blanks were sheared transverse to the rolling direction, and two blanks were sheared at 45 degrees from the rolling direction (diagonal). These blanks were then machined into test specimens, which were tension tested at room temperature and their n values determined. The values of each steel were then calculated. The n values in the longitudinal, transverse, and diagonal directions for the steels investigated are shown in Table II. Also shown in this table are the E values for these steels.

TABLE I [Compositions of ferritic stainless steels investigated, in percent] Heat No. 0 Mn Si Cr Mo Ch Note.-The above analysis reports only those elements which bear a relation to the instant invention. The various heats also contained:

TABLE II [The longitudinal, diagonal, transverse, and average strain-hardening exponents of the ierritic stainless steels investigated] Analysis of the E values in Table II indicated that a relation existed between T and the compositions of the steels investigated. It was found that the relation could be expressed by the following equation:

Inspection of this equation indicates the complexity of the problem of attempting to tailor the cold-heading properties (E) of ferritic stainless steels. The quadratic elfect of carbon and the interaction between carbon and chromium further complicate the problem. Although Cb also has a pronounced quadratic effect on if present in an appreciable degree, it was found that the equation could be somewhat simplified for the normal range of Cb as an incidental steelmaking impurity, e.g., 0.01 to 0.04%. However, with the aid of this equation, certain compositions, within the broad range below, can be produced to exhibit unexpectedly improved cold-heading characteristics, as represented by E values less than 0.17.

The effects of each of the elements, within the range (in weight percent) C trace to 0.15%. Mn trace to 2.5%. Si trace to 2.0%. Cr 14 to 21%.

Mo trace to 2.0%.

is discussed below.

The consequence of the carbon-chromium interaction on is shown in FIG. 1, at four different levels of chromium. When carbon is substantially below 0.09%, an increase in the chromium content will lower the r? value of the steel, whereas when carbon is substantially above 0.09 percent, an increase in chromium will be deleterious to cold-heading properties. Thus, carbon and chromium may be employed inversely to each other to provide two different compositions, both of which exhibit desirably low values, one with high chromium and low carbon and the other with low chromium and high carbon.

Perusal of the equation, shows that molybdenum and manganese are both beneficial with respect to cold-heading characteristics. In a steel in which molybdenum is only present as an incidental residual impurity (e.g.,

about 0.1 percent), at least about 1.4% Mn should be employed to insure a desirably low value. However, since Mn has a marked effect on strengthening of the ferrite phase, which in turn will be deleterious to cold headability, no more than about 2.5% Mn should be employed. Silicon, on the other hand, is detrimental to low values and should generally be limited to a maximum of 0.2 percent.

If molybdenum is employed as a purposeful alloy addition, then a somewhat lower amount of manganese and higher limit of silicon can be tolerated. However, unless molybdenum is employed for its known effects (i.e., to impart resistance to pitting corrosion) it will be more economical to achieve low E values by employing manganese near the higher end of the range and silicon at the lower end.

Therefore, ferritic stainless steels with enhanced coldheading characteristics, as represented by E values less than about 0.17 may be obtained in compositions comprising, in weight percent,

Composition 1 C under 0.09.

Cr 19.0 to 21.0.

Mn 1.5 to 2.5.

Si 0.20 max.

Mo 0.5 to 2.0 (optional).

balance Fe and incidental residual impurities, or

Composition 2 C greater than 0.09.

Cr 14.0 to 17.0.

Mn 1.5 to 2.5.

Si 0.20 max.

M0 0.5 to 2.0 (optional).

balance Fe and incidental residual impurities.

Thus, if in the design of ferritic stainless steel fasteners, cost factors were of primary concern, one would look to composition 2 for production of the fastener rod, wire or other stock material, and in which composition molybdenum is only present as an incidental steelmaking impurity. Carbon would then be employed at the high end of the range and chromium at the low end of the range. On the other hand, if corrosion resistance were of primary concern, then composition 1 would be employed, with high chromium, high molybdenum, and low carbon.

I claim:

1. A method for enhancing the cold-heading characteristics of ferritic steels of the type consisting of C trace to 0.15%.

Mn trace to 2.5%. Si trace to 2.0%.. Cr 14.0 to 21.0%. Mo trace to 2.0%.

balance Fe and incidental residual impurities which comprises combining the aforesaid elements in proportions in accord with the equation creased by employing more than about 1.5% Mn.

5. The method of claim 1 wherein the corrosion resistance is enhanced While achieving a low Evalue, which comprises employing from about 19.0 to about 21.0% Cr in conjunction with substantially less than 0.09% carbon.

6. The method of claim 5 wherein about 1.5 to 2.5% Mn is employed to further enhance the cold-heading characteristics of the steel.

7. The method of claim 6 which comprises employing less than about 0.2% Si.

8. The method of claim 7 wherein said H value is further decreased and said corrosion resistance is further increased by employing from about 0.5 to about 2.0% Mo.

9. A method for producing ferritic stainless steel fasteners which comprises combining alloying elements within the range, by weight percent C trace to 0.15%. Mn trace to 2.5%. Si trace to 2.0%. Cr 14.0 to 21.0%. Mo trace to 2.0%.

balance Fe and incidental residual impurities to produce an alloy in amounts and proportions which satisfy the equation C trace to 0.15%. Mn trace to 2.5%. Si trace to 2.0%. Cr 14.0 to 21.0%.

Mo trace to 2.0%.

balance Fe and incidental residual impurities in which the aforesaid elements are combined in proportions which satisfy the equation 11. The steel article of claim 10 consisting of C substantially greater than 0.09%. Mn 1.5 to 2.5%.

Si trace to 0.2%.

Cr 14.0 to 17.0%.

balance Fe and incidental residual impurities.

12. The steel article of claim 11 consisting of C substantially less than 0.09%. Mn 1.5 to 2.5%.

Si trace to 0.2%.

Cr 19.0 to 21.0%.

13. The steel article of claim 12. additionally containing from about 0.5 to 2.0% Mo.

References Cited UNITED STATES PATENTS 1,745,360 2/1930 De Silva 75-128 W 2,009,974 7/1935 Payson l48135 X 2,051,415 8/1936 Payson 75-128 W X 2,141,016 12/1938 Payson 75128 W X OTHER REFERENCES K. I. Irvine and F. B. Pickering, Low-carbon Steels with Ferrite-Pearlite Structures, Jnl. of The Iron and Steel Institute, November 1963, pp. 944-959..

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 

